1. Field of the Invention
The present invention relates to an uplink packet transmission method in a mobile communication system, and more particularly, to a method of performing uplink packet transmission in a mobile station communicating with at least two base stations generating ACK/NACK signals according to a hybrid automatic repeat request (HARQ) scheme.
2. Discussion of the Related Art
Hybrid automatic repeat request (HARQ) scheme applies channel coding to an automatic repeat request (ARQ) scheme. In ARQ scheme, a transmitting end transmits packet to a receiving end, and receives an acknowledgement (ACK) or negative acknowledgement (NACK) according to status of receiving and decoding in the receiving end.
Therefore, in a mobile communication system supporting a HARQ scheme, the receiving end, e.g., a base station, generates the ACK/NACK signal to indicate a packet transmission acknowledgment status based on the reception and decoding status of a corresponding packet, so that the transmitting end, e.g., a mobile station or user equipment (UE), is conditionally informed of the need to retransmit reverse traffic data. That is, the base station transmits an ACK signal if retransmission is unnecessary, i.e., when the transmitted signal is successfully received and decoded, and transmits a NACK signal if retransmission is necessary, i.e., when the transmitted signal is unsuccessfully received or decoded.
A HARQ system stores a previously received packet in which decoding has failed, combines the stored packet with a retransmission packet which is retransmitted in response to a NACK signal, and then performs an additional decoding operation on the combined resultant. In doing so, diversity gain and coding gain can be increased over a system. In addition, while the HARQ system uses physical layer signaling in transmitting an ACK/NACK signal and therefore exhibits minimal transmission delay, the ARQ system suffers from higher transmission delay times since ACK/NACK signals are transmitted by upper layer signaling.
HARQ is also applicable to a case of uplink packet transmission as well as downlink packet transmission. When a user equipment (UE) communicates with several base stations, such as in a soft handover condition, base station decides a presence or non-presence of error occurrence for a transmitted packet and then transmits a downlink ACK/NACK signal. In doing so, if each of the base stations transmits the ACK/NACK signal to the user equipment, the user equipment detects the ACK/NACK signal for each of the base stations.
The user equipment determines ACK or NACK for each of the base stations respectively, but uplink power control is performed with the combined power from all of the base stations. Therefore, each downlink reception power or signal to Interference ratio (SIR) may vary unstably, and the transmitting power for ACK/NACK signals needs high value excessively.